The present invention relates to a developing device for developing a static latent image on an image carrier, by bringing a developing unit of a respective one of difference colors in contact on a belt-like image carrier.
For example, as an image forming apparatus for forming a color image on a sheet, the type disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-211984 is conventionally known.
Such a device includes four development units, a photosensitive belt and an intermediate transfer belt. With this device, when a color image is formed on a sheet, a respective one of the four developing units selectively is brought into contact with the photosensitive belt, and thus a static latent image formed on the photosensitive belt is developed. Then, the toner image developed on the photosensitive belt is transferred onto the intermediate transfer belt. The just-described operation is repeated for each of the colors and thus toner images of these different colors are formed one after another on the photosensitive belt. Then, thus formed toner images are transferred onto the intermediate transfer belt consecutively, and an overlaid toner image of images of these colors is transferred in batch onto a sheet. The toner images of these colors transferred on the sheet are allowed to pass through a fixing device, where the images are fixed onto the sheet. Thus, a desired color image is formed on the sheet.
The contact-enabling mechanism designed to bring the developing units of the four colors selectively into contact with the photosensitive belt, has two stepping motors. One of these stepping motors serves to bring two developing units selectively into contact with the photosensitive belt, whereas the other stepping motor serves to bring the rest of the two developing units selectively into contact with the photosensitive belt.
More specifically, one stepping motor is rotated from its initial position in a predetermined direction by an angle of 120xc2x0 so as to bring one developing unit into contact with the photosensitive belt, and it is rotated from its initial position in an opposite direction to the above by an angle of 120xc2x0 so as to bring the other developing unit into contact with the photosensitive belt. Similarly, the other stepping motor is rotated from its initial position in both of forward and backward directions by an angle of 120xc2x0 so as to bring the other two developing units selectively into contact with the photosensitive belt.
However, in order to bring one developing unit selectively into contact with the photosensitive belt by the contact-enabling mechanism, it is required to control the rotation direction, rotation angle, rotation timing and the like of two stepping motors while associating them with each other, and therefore such a contact-enabling mechanism becomes complicated in structure as well as controlling operation, which naturally causes an increase in the product cost for the apparatus.
Further, according to the above-described technique, a respective stepping motor is rotated at once in a particular direction by an angle of as large as 120xc2x0 for bringing a respective developing unit into contact with the photosensitive drum, and therefore it takes a lot of time for the stepping motor to rotate, thereby decreasing he switching speed of the developing unit. On the other hand, if the rotation speed of the stepping motor is increased to increase the unit switching speed, the contact speed of the developing unit onto the photosensitive belt is increased, thus making a great impact on the belt. This impact causes the photosensitive belt to vibrate, thereby possibly creating a problem such as blurring of the image.
The present invention has been proposed in consideration of the above-described drawbacks and an object thereof is to provide a developing device having a contact-enabling mechanism for bringing a plurality of developing units selectively into contact with a photosensitive belt, formed in a simplified structure to be simply controllable, thereby making the production cost for the device.
Further, another object of the present invention is to provide a developing device which can increase the speed of the switching of a plurality of developing units by the contact-enabling mechanism, and at the same time, can decrease the contact speed of each of the developing units onto the photosensitive belt, thereby preventing drawbacks innate to the conventional technique, such as blurring of an image.
In order to achieve the above-described objects, there is provided according to one aspect of the present invention, a developing device comprising: a plurality of developing units for supplying a developing agent on a latent image on an image carrier, so as to develop the latent image; and a contact-enabling mechanism, having a plurality of cams for bringing the plurality of respective developing units into contact with the image carrier, one driving source for rotating the plurality of cams and one clutch for transmitting a driving force from the driving source to the plurality of cams so as to rotate these cams at the same time by a predetermined angle of degrees, the contact-enabling mechanism selectively bringing one of the plurality of developing units in contact with the image carrier each time the clutch is set on.
Further, there is provided according to the present invention, a developing device comprising: four developing units for supplying developing agents of respective colors on latent image of respective colors formed on an image carrier, so as to develop the latent images on the basis of color-separated 4-color image signals; and a contact-enabling mechanism, having four cams for bringing the four respective developing units into contact with the image carrier, one driving source for rotating the four cams and one clutch for transmitting a driving force from the driving source to the four cams so as to rotate these cams at the same time by an angle of 72 degrees, the contact-enabling mechanism selectively bringing one of the four developing units in contact with the image carrier each time the clutch is set on.